1. Field of the Invention
This invention relates generally to portable or transportable electrical power supply systems such as fuel driven generators and batteries and more particularly relates to a hybrid combination of a power generating system having an alternator driven by a Stirling engine and a storage battery or other device for storing electrical energy.
2. Description of the Related Art
An extensive variety of electrical power supply systems have been known in the prior art for well over a century for powering the many electrical devices available for assisting human effort. Portable systems offer the opportunity to use such electrical devices in remote areas where electrical power is unavailable or inconvenient.
Some of the most common portable electrical power sources are storage batteries and they come in an extensive variety of designs. The term battery is used herein to generically include rechargeable storage batteries and other rechargeable electrical energy storage devices. For example, such electrical energy storage devices would also include capacitor banks or rechargeable fuel cells. Where there is a need for a longer term supply of electrical power, power supply systems are required that can generate electrical power from the energy in available fuels and apply that power to both recharging batteries and to meet the power demands of the various user loads presented by the electrical devices needed by the user.
It is well known in the art to connect a user load to an electrical power generating system or, alternatively, to connect a battery to the generating system in order to charge the battery when it has become discharged. However, many such systems require changing the manual connection of the load, generating system and battery each time the mode of operation is changed between the modes of the generating system driving the load, the generating system charging the battery and the battery driving the load.
Another problem inherent in all electrical power generating systems is that they have a maximum output power at which they can maintain an output voltage that at least meets the minimum output voltage required by the user load. Because of the internal resistance of all generators and batteries, increased power delivery causes a decline in output voltage. Eventually, as output power increases, electrical output power exceeds the power at which a selected minimum output voltage can be maintained and output voltage drops below the minimum voltage required by the user load. The load must be disconnected from the generating system or its load demand reduced in order to permit it to continue operating and/or to avoid damage to the user load. As a result, the power supplied to the user load is limited to the maximum power output that the generating system can deliver. This limitation is particularly restrictive for a user load that, only for relatively brief time intervals, exceeds the power delivery capacity of the generating system.
One portable electrical power generating system that offers particular advantages is the combination of a Stirling engine driving a linear alternator that charges a battery. In such prior art systems, (1) DC power is available from the battery, (2) AC and/or DC power are available from the alternator, the DC implementation requiring an intermediate rectifier, and (3) the alternator can be used for charging the battery.
As with most portable devices, smaller size and weight are desirable characteristics in order to facilitate transportation and handling. Another desirable characteristic for an electrical power generating system is a high peak power delivery capacity for the reasons described above. Although electrical power generating systems can be designed to provide nearly any desired power delivery capacity, with portable generating systems the power generating capacity is inversely related to the size and weight of the system which presents a design tradeoff challenge.
Different electrically powered devices not only present different load demands to an electrical power generating system, but the load demand of many devices can vary, often widely, during use as a result of variations in operating conditions. As a result, there are often time intervals of operation during which the power generating system may be connected to a load that present an unusually high power demand for a relatively short time interval. Conventionally, portable electrical power supply systems are designed to deliver a maximum power equal to the expected peak power. This means that under much, and ordinarily most, of its operating lifetime, the power supply system will be supplying power below this maximum capacity with the remaining capacity being in reserve to meet the temporary peak power demand. Consequently, with this conventional design, for 100% of its operating lifetime, the portable generating system is larger and has a greater weight than is needed most of the time. Only during the relatively short time intervals of peak power demand does it utilize the full benefit of 100% of its weight and size.
It is a purpose and feature of the invention to provide an apparatus and method for operating an electrical power supply system that is a hybridized combination of a Stirling engine driving a linear alternator that can charge a battery, but is also able to deliver a peak power to a load even when the peak power demand exceeds the maximum power that the alternator is able to supply under long term steady state conditions. This allows the load demand of the user's load to exceed the power output maximum of the Stirling generating system for a limited, though extensive, period of time. For example, the invention permits an electrical power supply system of this type to be designed with an engine/alternator combination that has a maximum capacity for continuous delivery of 130 We but the hybrid electrical power supply system is capable of delivering 230 We for up to 30 minutes. This allows the engine/alternator to have only a size and weight that makes it capable of delivering up to 130 We rather than the increased size and weight necessary to deliver up to 230 We.